1. Technical Field of the Invention
The present invention relates to systems and methods for controlled disconnect of a surface vessel from a subsea well, and more particularly, to such a system and method that prevents release of fluids from the conduit into the sea when the conduit is disconnected.
2. Description of Related Art
In operations such as well testing, clean-up, perforating, or other similar operations, a vessel at the sea surface is connected to the wellhead by both a riser and a tubular working string. The position of the vessel is controlled so that the vessel resides over the wellhead to maintain the connection. If the vessel must move away or drive-off from the subsea well, the connection between the vessel and the subsea well must be severed to prevent damaging the vessel, the working string, and the riser. Additionally, the well must be shut-in to prevent a blowout of well fluids, which unfortunately, would be channeled up the riser towards the vessel.
A drive-off may result from several situations. For example, with a dynamically positioned vessel, one or more components of the dynamic positioning system can malfunction and cause the relative position of the vessel and well to suddenly change. A vessel that is held in place by tensioned cables may be propelled away from the well if one of the tensioned cables breaks. Also, the drive-off may be intentional, for example, to avoid a bad weather system.
In conventional systems, the wellhead provides a profile that receives a tubing hanger. The tubing hanger, in turn, supports the working string. The working string may incorporate a retainer valve above a subsea test tree that is actuable to allow or prevent flow through the working string. A blow-out preventer (BOP) stack is provided on the casing at the wellhead, and is actuable to seal the annulus between the working string and the casing.
In normal operations, fluid is communicated between the vessel and well through the working string. The annulus between the working string and the casing is sealed by a packer. In the event of a drive-off, the working string is separated at the wellhead, and the BOP stack seals the annulus. The working string above the wellhead or subsea test tree can then be pulled from the riser, and the working string below the wellhead or subsea test tree is supported in the well by the tubing hanger.
More recently, however, well systems have incorporated a continuous diameter casing and riser with the BOP stack positioned either near the vessel or intermediate the vessel and the sea floor. With such systems, a conventional working string configuration as described above cannot be used, because there is no profile for the tubing hanger to engage or BOP stack to isolate the annulus at the seabed. Thus, in operations, the entire working string is supported from the vessel. In the event of a drive-off, the working string would be pulled from the well as the vessel departs. If the working string were configured to separate, the lower portion of the string would drop unsupported into the well, because there is no tubing hanger to provide vertical support. Additionally, the BOP stack positioned near the vessel or intermediate the vessel and sea floor is above the usual point of separation at the seabed. Consequently, if the work string is parted, the entire volume of the riser above the seabed is exposed to pressurized well effluent which may be released to the environment if the riser is parted or ruptures, alternatively, released gas may evacuate the riser above the seabed and expose it to high collapse pressures which may cause failure.
Therefore, there is a need for a system and method for use in well operations that does not require the working string be supported by a tubing hanger in the event of a drive-off or other situation requiring separation of the working string. Further, the system should seal the annulus between the casing and the working string when the working string is separated.